This document specifies methods used to determine the concentration of plutonium and neptunium isotopes in water by inductively coupled plasma mass spectrometry (ICP-MS) (239Pu, 240Pu, 241Pu and 237Np). The concentrations obtained can be converted into activity concentrations of the different isotopes.
Due to its relatively short half-life and 238U isobaric interference, 238Pu can hardly be measured by this method. To quantify this isotope, other techniques can be used (ICP-MS with collision-reaction cell, ICP-MS/MS with collision-reaction cell or chemical separation). Alpha spectrometry measurement, as described in ISO 13167, is currently used.
This method is applicable to all types of water having a saline load less than 1 g·l-1. A dilution of the sample is possible to obtain a solution having a saline load and activity concentrations compatible with the preparation and the measurement assembly.
A filtration at 0,45 µm is needed for determination of dissolved nuclides. Acidification and chemical separation of the sample are always needed.
The limit of quantification depends on the chemical separation and the performance of the measurement device.
This method covers the measurement of those isotopes in water in activity concentrations between around:
— 1 mBq·l-1 to 5 Bq·l-1 for 239Pu, 240Pu and 237Np;
— 1 Bq·l-1 to 5 Bq·l-1 for 241Pu.
In both cases, samples with higher activity concentrations than 5 Bq·l-1 can be measured if a dilution is performed before the chemical separation.
It is possible to measure 241Pu following a pre-concentration step of at least 1 000.